The role of interfacial donor–acceptor percolation in efficient and stable all-polymer solar cells

Polymerization of Y6-type acceptor molecules leads to bulk-heterojunction organic solar cells with both high power-conversion efficiency and device stability, but the underlying mechanism remains unclear. Here we show that the exciton recombination dynamics of polymerized Y6-type acceptors (Y6-PAs) strongly depends on the degree of aggregation. While the fast exciton recombination rate in aggregated Y6-PA competes with electron-hole separation at the donor–acceptor (D–A) interface, the much-suppressed exciton recombination rate in dispersed Y6-PA is sufficient to allow efficient free charge generation. Indeed, our experimental results and theoretical simulations reveal that Y6-PAs have larger miscibility with the donor polymer than Y6-type small molecular acceptors, leading to D–A percolation that effectively prevents the formation of Y6-PA aggregates at the interface. Besides enabling high charge generation efficiency, the interfacial D–A percolation also improves the thermodynamic stability of the blend morphology, as evident by the reduced device “burn-in” loss upon solar illumination.

From femtosecond transient absorption studies, the authors find that hole transfer from the acceptor to the donor occurs on the tens of picoseconds time scale, justifying the need for nanosecond exciton lifetimes to support near unity exciton dissociation yields.From transient photoluminescence, they find that polymer acceptors in their solid state show strong aggregationinduced quenching and therefore can't sustain nanosecond exciton lifetimes.However, due to good miscibility with PM6, the polymer acceptors avoid aggregation at the donor-acceptor interface, thus avoiding aggregation-induced quenching.The good miscibility of the polymer acceptors with PM6 also reduces the trend for donor:acceptor demixing, causing electrical performance degradation in organic solar cells comprising PM6:Y6 blends.
There is no doubt that this study is timely and the objective -enacting control over morphology to match efficiency and stability in organic photovoltaics -is of high importance to the field and to society.However, in my opinion, both the generality and the validity of the key claims have not been sufficiently supported by evidence to demonstrate new design principles.For this reason, I do not think the manuscript is publishable in Nature Communications in its present form.On one hand, the results might be valid only for blends with PM6; on the other hand they might be valid beyond all-polymer solar cells and refer also to small molecule acceptors.I suggest a clarification of the scope and validity of the key claims before this paper can be published in Nature Communications.

Detailed analysis:
[1] Figure 1e: the HOMO and LUMO wavefunctions of PY-IT show very little spatial overlap; therefore the contribution of this configuration to the transition moment of the S0-S1 transition should be rather low.For a fair comparison with the small molecule Y6, I suggest to represent the configuration with the highest contribution to the transition moment of the S0-S1 transition.
[2] Page 9, line 169: The authors find strong aggregation-induced quenching (increase of knr) in the polymer Y6 while small molecule Y6 does not show this effect.Although all three polymer Y6 show this effect, the molecular structures show that all polymer Y6 contain tert-butyl groups.The authors should comment whether the notion of aggregation-induced quenching is intrinsic to polymer acceptors, or whether the phenomenon would need to be handled also in side chain engineering for small molecule acceptors.
[4] page 10, line 191.The authors show average numbers for contacting D-A pairs of 2.68 and 3.27 for PM6:Y6 and PM6:PY-IT, respectively, after accounting for the difference in molecular weights.However, in the supporting information, it has not become clear how this normalization to molecular weights has been achieved.Could the authors clarify this?
[5] Does a difference of less than 20% between the contacting numbers of PM6:Y6 and PM6:PY-IT justify the notion of an entirely different morphology (aggregated versus intercalated, respectively)?
[6] Figure 4c: according to this figure, the polymer acceptor shows better mixing with PM6 than small molecule Y6.Can this notion be generalized to other donor polymers than PM6?Is the better mixing due to the acceptor being a polymer, or due to the tert-butyl groups present in the polymer acceptors?
[7] Figure 4d: in this figure panel, the authors depict a "Y6-PA" phase on green background.However, according to Figure 2c, such a pure Y6-PA phase should present fast exciton quenching; excitons created in the pure Y6-PA phase would have low probability to diffuse to the interface before being quenched.However, in the absence of such a pure Y6-PA phase, typically one would expect fast exciton breaking but also fast geminate recombination.Indeed, Fig. 2b seems to indicate fast polaron loss after 100 ps.But that would contradict the notion of high efficiency, indicated in the title of the manuscript.Could the authors resolve these apparent contradictions?
[8] From Figure 4c, the author infer a greater stability of polymer acceptors against morphological degradation.However, morphological degradation does not only involve the donor-acceptor blend but also the extracting layers.Different interactions can lead to vertical demixing or changes of local alignment.Could the authors comment on the role of polymer acceptors with respect to these aspects of morphological degradation?
Reviewer #3 (Remarks to the Author): Please refer to the attached file Reviewer #3 A achment on the following page

Reviewer #1:
In this manuscript, Wang and co-workers investigate the charge generation mechanism and structure-property relationship of three high-performance all-polymer OPV blends based on polymerized-Y6 acceptors (Y6-PAs), and draw comparison to those found in blends based on the model small-molecule acceptor Y6.The authors used transient absorption spectroscopy to show that free charge generation happens at ~100 ps, and therefore a long (nanosecond) exciton lifetime in the low-gap acceptors is needed to facilitate efficient charge generation.While insufficient exciton lifetime is found in Y6-PA films, much extended exciton lifetime is found when the polymers are dispersed, indicating that dispersed Y6-PA chains are located at the percolated donor-acceptor interface.This picture is well supported by molecular dynamics (MD) simulations as well as resonant soft X-ray scattering (R-SoXS) experiments.Besides providing insights on the charge generation dynamics in Y6-PA blends, the results show that the all-polymer blends, thanks to the higher donor-acceptor miscibility, are much closer to the percolation threshold than those based on Y6 molecular acceptors.This in turn leads to a greater thermodynamic stability of the blend, which is known to be a major challenge for OPV based on Y6-type small-molecule acceptors.
Overall, this is a timely and solid study which will surely help the community to better understand the working mechanism of all-polymer solar cells, and inspire researchers especially those who work on material synthesis and device engineering to design more efficient and stable organic solar cells.The scope and novelty of this manuscript is suitable to Nature Communications, and the conclusions are well supported by the experimental and simulation results.I would be happy to recommend this manuscript be accepted for publication after some minor revisions and discussions.My detailed comments and suggestions are as follows: Author response: Many thanks for the reviewer's positive comments.
1.While the authors found that charge generation takes ~100 ps under 750 nm excitation, which selectively excites the acceptors, it is not clear if similar charge separation dynamics are found under other excitation wavelengths (e.g., when the donors are selectively excited).This is important since both donor and acceptor are excited under device operation.The authors should provide additional transient absorption data under other excitation wavelengths to cover this point.

Author response:
Thanks for pointing this out.Indeed, both donor and acceptor are excited under PV device operation.We have performed additional transient absorption (TA) data for the blended films under 550 nm excitation, which excites both donor and acceptor.It is found that there is little difference in the overall charge generation kinetics compared to the 750 nm excitation (both take ~100 ps to complete).This can be seen from the TA spectra (visible region) of the four blended films excited at 550 nm shown in Figure R1 below, and the decay kinetics of the ground state bleaching (GSB) of PM6 are summarized in Figure R2.Though the relative intensities are different in these blended films, the decay kinetics are similar, with the growth in polaron photoinduced absorption (charge generation) taking ~100 ps to complete for all four blends.We have now added Figure R1-R2 to the revised Supplementary Information as new Supplementary Figure S11-S12 and added the relevant discussion to the revised manuscript.2. The composition variation (domain purity) information was extracted from R-SoXS data acquired at 284 eV.While it seems that R-SoXS profiles acquired at 284.7 eV show higher intensities/contrasts .The authors should clarify this and compare the composition information extracted from data acquired at different energies.

620-650 nm
Thanks for the suggestion.The X-ray energy (284 eV) used for analysis in the manuscript is closer to the carbon K-edge (~283.8eV).Therefore, R-SoXS profiles acquired at this energy have better material contrasts and lower fluorescence signals than those at higher energies (J.Mater. Chem. C 2013, 1, 187-201;Phys. Rev. Lett. 2017, 119, 167801).
In order to check the energy-dependence of R-SoXS data, we further analysed R-SoXS profiles acquired at 284.7 eV and summarized in Figure R3 below.By normalizing to the data for PM6:Y6 film (taking it as 1), it is estimated that the root-mean-square composition variations HPR <9-0<B%7?$<9-0<B5%?%PCOF <9-0<B%A%`HKMNS CRG '&..$ '&.. COF '&/)$ RGSQGETKVGMY& This is fairly similar to the data we got at 284 eV, which again indicates that PM6:Y6 blend has better domain purity than the other all-polymer blends.3.According to previous report by Brabec et al. (Nat.Energy 2020, 5, 711-719), a long exciton lifetime is very important to allow efficient charge generation at the D-A interfaces in systems with small or negligible donor-acceptor energy offsets.However, the energy level offsets of the studied PM6:Y6-PA systems are not provided in the manuscript.The authors should include additional discussions in the manuscript to address this point.
The HOMO offset of these all-polymer blends falls into the charge-transfer-state-exciton (CT-LE) equilibrium regime that requires long exciton lifetime for decent external quantum efficiency (EQE), as described and discussed in the study mentioned by the reviewer (Nat. Energy 2020, 5, 711-719).The relationships of non-radiative energy loss, EQE and HOMO offset are shown in Figure R4.Moreover, though HOMO offset of these all-polymer blends is slightly larger than that of PM6:Y6 blend (0.09 eV, Joule 2019, 3, 1140-1151), our TA data indicates that it takes the same timescale (~100 ps) for charge generation to complete for PM6:Y6 and the three all-polymer blends.This implies that the slightly larger HOMO offset doesn't help much with the charge generation process, and hence a long exciton lifetime is necessary for these all-polymer blends as well.We have added relevant discussion in our revised manuscript to address this point.4. It is noted that some recent studies indicated that the choice of donor polymers can also play a key role on the blend morphology stability, e.g., Joule 2023, 7, 810-829.The authors should add some discussions about this point in the manuscript.

Author response:
Thanks for the comment, we agree that the choice of donor polymer is also an important factor for the overall morphology stability.We have added the relevant discussion in our revised manuscript, and in Supplementary Note 2 of the revised Supplementary Information for extended discussions.
Added discussion in main text: "Nevertheless, it should be noted that besides blend morphology stability, other factors such as polymer donor degradation and interfacial defects are also likely to affect the overall device stability.Therefore, in addition to developing strategies to improve morphological stability, future research should also target to overcome these factors in order to achieve highly stable and efficient OPV devices that can meet industry requirements." Reviewer #2: Zhen Wang et al. present a study of exciton dissociation dynamics and morphology in allpolymer organic solar cells.As acceptor materials, they used polymer derivatives of the small molecule acceptor Y6, that have been shown to combine high efficiency with enhanced stability against morphological degradation in previously published works.These polymer acceptors are blended with the PM6 donor polymer; comparison with the "standard" PM6:Y6 blend is also given in all cases.
From femtosecond transient absorption studies, the authors find that hole transfer from the acceptor to the donor occurs on the tens of picoseconds time scale, justifying the need for nanosecond exciton lifetimes to support near unity exciton dissociation yields.From transient photoluminescence, they find that polymer acceptors in their solid state show strong aggregation-induced quenching and therefore can't sustain nanosecond exciton lifetimes.However, due to good miscibility with PM6, the polymer acceptors avoid aggregation at the donor-acceptor interface, thus avoiding aggregation-induced quenching.The good miscibility of the polymer acceptors with PM6 also reduces the trend for donor:acceptor demixing, causing electrical performance degradation in organic solar cells comprising PM6:Y6 blends.
There is no doubt that this study is timely and the objective -enacting control over morphology to match efficiency and stability in organic photovoltaics -is of high importance to the field and to society.However, in my opinion, both the generality and the validity of the key claims have not been sufficiently supported by evidence to demonstrate new design principles.For this reason, I do not think the manuscript is publishable in Nature Communications in its present form.On one hand, the results might be valid only for blends with PM6; on the other hand, they might be valid beyond all-polymer solar cells and refer also to small molecule acceptors.I suggest a clarification of the scope and validity of the key claims before this paper can be published in Nature Communications.

Author response:
Many thanks for the reviewer's overall positive comments.The suggestions from the reviewer surely help a lot to improve our work.In the revision manuscript, we have included additional experimental data and discussions to address the reviewer's comments.

Detailed analysis:
[1] Figure 1e: the HOMO and LUMO wavefunctions of PY-IT show very little spatial overlap; therefore, the contribution of this configuration to the transition moment of the S0-S1 transition should be rather low.For a fair comparison with the small molecule Y6, I suggest to represent the configuration with the highest contribution to the transition moment of the S0-S1 transition.

Author response:
Thanks for pointing this out.By further analysing our DFT results, it should be noted that while the S0--S1 transition for Y6 is indeed coming from HOMO to LUMO orbital transition, for Y6-PAs, there are multiple orbital transitions that contribute to S0--S1 transition.For PY-IT and PYF-T-o, we find that HOMO-1 to LUMO contributes mainly to the S0--S1 transition, and for <B%A%`$ 6;9; TP 8@9; EPOTRKDUTGS NCKOMY TP TJG >0--S1 transition along with others.We have summarized the orbital transitions that contribute to S0--S1 transition of the four acceptors in Table R1 below.Also, we have obtained the HOMO-1 orbitals and added them in Figure 1 (with updated figure caption and main text), as well shown in Figure R5 below.
Table R1.Orbital transitions that contribute to S0--S1 transition of four acceptors.[2] Page 9, line 169: The authors find strong aggregation-induced quenching (increase of knr) in the polymer Y6 while small molecule Y6 does not show this effect.Although all three polymer Y6 show this effect, the molecular structures show that all polymer Y6 contain tertbutyl groups.The authors should comment whether the notion of aggregation-induced quenching is intrinsic to polymer acceptors, or whether the phenomenon would need to be handled also in side chain engineering for small molecule acceptors.

Author response:
Thanks for the insightful comment.To reveal whether the aggregation-induced quenching is intrinsic to these polymer acceptors, as opposed to the choice of side chains, we have introduced an additional material system in our study: PY-monomer.As shown in the chemical structure (Figure R6), PY-monomer has the same side chains as the Y6-PAs studied in this work.Figure R7 shows the PL and TRPL data for the PY-monomer, and Figure R8 and R9 show the TA data for PY-monomer neat and blended films.It is clear that the PY-monomer behaves very similarly to Y6, even though they have different side chains.This provides evidence that the aggregation-caused quenching comes from the polymerization of these systems.We have added the relevant data (as new Supplementary Figure S26-29) and discussions in the revised main text and Supplementary Information.
Added discussion in main text: "We also performed TA and TRPL for the PY-monomer, which has the same side chains as Y6-PAs studied in this work (see Supplementary Fig. S26-29).We observe little difference in exciton dynamics between the PY-monomer and Y6 films, which implies that the changes in exciton dynamics observed in the Y6-PAs are results of polymerization rather than the side chains."[3] Page 9, line 181: "near acceptor films": do the authors mean "neat acceptor films"?

Author response:
Thanks for pointing this out.Yes, we meant "neat acceptor films", and the typo is now corrected in the revision manuscript.
[7] Figure 4d: in this figure panel, the authors depict a "Y6-PA" phase on green background.However, according to Figure 2c, such a pure Y6-PA phase should present fast exciton quenching; excitons created in the pure Y6-PA phase would have low probability to diffuse to the interface before being quenched.However, in the absence of such a pure Y6-PA phase, typically one would expect fast exciton breaking but also fast geminate recombination.Indeed, Fig. 2b seems to indicate fast polaron loss after 100 ps.But that would contradict the notion of high efficiency, indicated in the title of the manuscript.Could the authors resolve these apparent contradictions?
Author response: Thanks for the insightful comment.First, we clarify that Figure 4d (now 4c) in the manuscript is just a schematic illustration of the (nanoscale) D-A mixing at the interface, and does not mean that the Y6-PA phase is excessively large and pure (in fact the opposite is concluded based on the presented R-SoXS and MD results).Based on the high efficiency (PCE >15% and EQE >75%), we know that vast majority of excitons can reach the D-A interface and separate.Indeed, we do see faster polaron loss after 100 ps in TA results, and we believe this is in line with lower FF of the Y6-PA blends.This is likely due to reduced domain purity, as we uncovered in R-SoXS results, slightly increasing recombination losses during charge transport.
We believe that future research should target to address this increase in recombination losses caused by the reduced domain purity.
Added discussion in main text: "The reduced domain purity that we revealed for the Y6-PA blends is consistent with their faster carrier decay (Fig. 2b) and lower device PCE (~13.2-15.5%)compared to Y6 blend (~16.0 %).Nevertheless, these Y6-PA blends are still among the most efficient all-polymer OPV systems to date, and we consider that high intra-chain electron transport in these materials helps to reduce recombination losses by allowing charges to move away from the mixed D-A interface.
Engineering of the intra-chain charge transport for both polymer donor and acceptor materials may provide a design pathway to further suppress recombination losses in all-polymer blends." [8] From Figure 4c, the authors infer a greater stability of polymer acceptors against morphological degradation.However, morphological degradation does not only involve the donor-acceptor blend but also the extracting layers.Different interactions can lead to vertical demixing or changes of local alignment.Could the authors comment on the role of polymer acceptors with respect to these aspects of morphological degradation?
Author response: Thanks for the comment.We agree that other factors are also important to the device stability.
To further investigate the structure-device relationship, also in response to Comment #6 by Reviewer #3, we have fabricated device samples and did controlled photostability testing with the same device architecture (which rules out the influence of the extracting layers), and it is clear the all-polymer systems show better photostability (less "burn-in") than the PM6:Y6 system thanks to the more stable D-A interfacial morphology.We have added additional device data and discussions to our revised manuscript and Supplementary Information.An in-depth investigation of all these factors is, however, beyond the scope of this present work.
Added discussion in main text: "For device stability, it is clearly shown that PM6:Y6 device suffered from a more rapid decay in efficiency upon solar illumination compared to the three PM6:Y6-PA blends.This rapid efficiency drop is attributed to the device "burn-in", which is often observed in non-fullerene SMA-based blends with hypo-miscible D-A morphology (over-purification) at the interfaces.Thanks to the increased D-A miscibility and interfacial percolation in the Y6-PA blends, these all-polymer blends suffer from reduced "burn-in" losses and thus an improved device stability over their SMA counterpart.We consider that the increased D-A miscibility in Y6-PA-based blends helps to explain the overall improvement in device stability compared to Y6-SMA-based blends (see Supplementary Note 3 for extended discussion).Nevertheless, it should be noted that besides blend morphology stability, other factors such as polymer donor degradation and interfacial defects are also likely to affect the overall device stability.Therefore, in addition to developing strategies to improve morphological stability, future research should also target to overcome these factors in order to achieve highly stable and efficient OPV devices that can meet industry requirements." Reviewer #3: The study conducted by Chow et al. described an intriguing investigation into the Y6-type small molecule acceptors (Y6-SMAs) and all-polymer blends based on polymerized Y6-type acceptors (Y6-PAs) in underlying their charge generation dynamics and structure-property relationships.The authors suggest that the well dispersed Y6-PA showed a much shorter longer exciton lifetimes (~1.1-1.9 ns) than aggregated polymers in films (~0.3-0.5 ns), taking the advantages of the aggregated polymers Y6-PAs have larger miscibility with the donor polymer (PM6) than Y6-SMAs, leading to D-A percolation that effectively prevents the formation of Y6-PA aggregates at the interface.Therefore, the interfacial D-A percolation plays a key role in suppressing interfacial charge recombination to enable efficient charge generation.
Overall, this manuscript is well-organized and well-written.However, some of the comments overclaim with insufficient experimental support.Major revisions and supplementary data are needed before we would consider recommending this manuscript for publication in Nature Communications.Specific comments are as follows.4. The authors mentioned that when these Y6-PAs are blended with PM6, a blue shift in emission was observed.However, the blue shift of these Y6-PAs is so small.Please further elaborate on their differences in terms of molecular packing, providing supportive evidence.
the emissive state lifetime is a very important that determines the charge generation efficiency at D-A interface (as pointed out by Brabec et al.).
6. Please fabricate the device show that the PCE and operation stability are indeed improved after all these characterisation and MD calculation.

Author response:
Thanks for the suggestion.We have fabricated devices and test their stability.The device performance of the four systems is summarized in Table R3, and the photostability test under 1 Sun illumination is displayed in Figure R22.Devices are fabricated in conventional structure with ITO/PEDOT:PSS/active-layer/PNDIT-F3N/Ag, and the efficiencies are comparable to the reported numbers in literature.As shown in Figure R22, PM6:Y6 system decays faster than the other PM6:Y6-PA systems, which could be attributed to the well-known phenomenon so called "burn-in".The all-polymer systems showed less "burn-in" than small-molecule system.As described previously, "burn-in" mainly comes from the over-purification of at the interfaces driven by the hypo-miscible nature of the blends (Nat. Mater. 2021, 20, 525-532).This confirms our conclusion that the high miscibility of PM6:Y6-PA blends could "lock" the interfacial morphology and stabilize the devices by suppressing "burn-in".We have revised our manuscript and Supplementary Information accordingly to include these new device data and discussions.
Added discussion in main text: "To study the structure-device property relationship, we fabricate device samples of PM6:Y6 and the three PM6:Y6-PAs blends and monitor the change in their PV performance under continuous solar illumination (see Supplementary Fig. S47 and Table S6).The reduced domain purity that we revealed for the Y6-PA blends is consistent with their faster carrier decay (Fig. 2b) and lower device PCE (~13.2-15.5%)compared to Y6 blend (~16.0 %).Nevertheless, these Y6-PA blends are still among the most efficient all-polymer OPV systems to date, and we consider that high intra-chain electron transport in these materials helps to reduce recombination losses by allowing charges to move away from the mixed D-A interface.
Engineering of the intra-chain charge transport for both polymer donor and acceptor materials may provide a design pathway to further suppress recombination losses in all-polymer blends.
For device stability, it is clearly shown that PM6:Y6 device suffered from a more rapid decay in efficiency upon solar illumination compared to the three PM6:Y6-PA blends.This rapid efficiency drop is attributed to the device "burn-in", which is often observed in non-fullerene SMA-based blends with hypo-miscible D-A morphology (over-purification) at the interfaces.Thanks to the increased D-A miscibility and interfacial percolation in the Y6-PA blends, these all-polymer blends suffer from reduced "burn-in" losses and thus an improved device stability over their SMA counterpart.We consider that the increased D-A miscibility in Y6-PA-based blends helps to explain the overall improvement in device stability compared to Y6-SMA-based blends (see Supplementary Note 3 for extended discussion).Nevertheless, it should be noted that besides blend morphology stability, other factors such as polymer donor degradation and interfacial defects are also likely to affect the overall device stability.Therefore, in addition to developing strategies to improve morphological stability, future research should also target to overcome these factors in order to achieve highly stable and efficient OPV devices that can meet industry requirements."

Figure R7 .
Figure R7.(a) Normalized PL spectra for PY-monomer solution/film samples and PM6:PY-monomer blend film.(b) TRPL data for PY-monomer solution and film samples.

Figure R9 .
Figure R9.Integrated decay kinetics for GSB features of PY-monomer (a) and charge generation (hole transfer) features of PM6:PY-monomer blend (b).